Which layer of the OSI model does a firewall operate on when filtering by TCP/UDP port number?

A firewall filtering traffic by TCP/UDP port number operates at Layer 4 of the OSI model, which is the Transport layer. This is where protocols like TCP (Transmission Control Protocol) and UDP (User Datagram Protocol) function. The Transport layer is responsible for end-to-end communication and reliability of data transfer, as well as the control of flow and error recovery. By examining the port numbers associated with TCP or UDP, a firewall can determine which application data is associated with the communication, making it possible to allow or block specific types of traffic based on defined rules.

The other layers mentioned—Layer 2 (Data Link), Layer 3 (Network), and Layer 5 (Session)—do not directly deal with TCP/UDP port filtering. Layer 2 focuses on physical addressing and frame transmission over a local network, Layer 3 is concerned with logical addressing and routing of data packets across networks, and Layer 5 manages sessions between applications. Thus, firewalls specifically filtering by port number function at Layer 4, where the distinctions of TCP and UDP packaging occur.